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Study findings indicate that cyanobacteria and cyanotoxins were not associated with mussel mortality at the concentrations present in Lake Erie during a recent study (2013-15), but mussel growth was lower at sites with greater microcystin concentrations.
Lake Erie experienced major changes in its cyanobacterial communities in the early 2000s. After being absent for most of the late 1980s and 1990s, cyanobacteria have again become prevalent seasonally in the western basin of Lake Erie, which provides drinking water to large coastal communities. These cyanobacterial blooms have resulted in episodic drinking water shutdowns along Lake Erie and the creation of a large international effort to identify causes and potential management strategies to minimize adverse effects to humans and other organisms.
Lake Erie supports an important commercial and cultural fishery. These fisheries rely in part on production (growth and survival) of mussels in the lake, and there is concern that cyanobacterial blooms and associated cyanotoxins could reduce production of mussels and ultimately affect the fishery.
Cyanobacteria produce several different classes of cyanotoxins, including anatoxin, cylindrospermopsin, microcystin, and saxitoxin. Microcystin is the only toxin known to regularly occur in Lake Erie. Mussels often are not affected by microcystin at concentrations that cause mortality of other organisms such as zooplankton. However, more recent studies have shown that microcystin can cause inflammatory responses and immune system alterations in dreissenid mussels when present either in dissolved form or in the cells of ingested cyanobacteria. This is of particular interest in Lake Erie, where invasive dreissenid mussels comprise the majority of secondary production.
Scientists assessed if cyanobacterial abundance indices and cyanotoxin concentrations were associated with secondary production by mussels in Lake Erie. Three indices of mussel production (growth of a native unionid mussel, the size of young-of-year invasive dreissenid mussels, and the overall mass of colonizing animals on a Hester-Dendy sampler) were measured, and indices of cyanobacterial abundance (biovolume, chlorophyll-a concentration, and cyanotoxin concentration) at 35 stations in the western Basin of Lake Erie during 2013 through 2015.
The results of this study indicate that cyanobacterial abundance and microcystin did not cause mussel mortality at the concentrations detected in Lake Erie during 2013-2015. Measures of cyanobacterial abundance were only weakly associated with the impaired growth of mussels. In contrast, mussel growth was lower at sites with greater microcystin concentrations. This information suggests that in Lake Erie, the poor food quality of cyanobacteria has only a small influence, while microcystin concentration has a larger influence, on mussel growth. Though this study largely reflects effects of cyanobacteria on mussels, rather than all secondary producers, the invasive dreissenid mussels make up a very high proportion of secondary production in the Western Basin of Lake Erie. These mussels do provide resources and a pathway for biomass to move to higher trophic levels in the food web.
This study is part of a larger USGS goal to provide the science needed to understand how to economically and effectively minimize the risk, if any, to the health of humans and other organisms exposed to toxins through inhalation, dermal, ingestion, and other exposure routes. Future steps include the need to understand the mechanisms responsible for reduced native mussel growth associated with microcystin and other cyanotoxins in our Nations water resources.
The USGS Toxic Substances Hydrology Program and the Great Lakes Restoration Initiative funded this study.
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